The present invention relates to a hot-rolled steel sheet of high cold formability, particularly to a hot-rolled steel sheet of high formability in relation to stretch flanging and bendability, and to a method of producing such steel sheet.
In order to develop a satisfactory steel sheet, particularly a hot-rolled steel sheet, the cold formability, particularly the formability in regard to stretch flanging and bendability, should be improved. Among others, the main factor of lowering such formability in respect to stretch flanging and bendability are stringer inclusions which are sulfide inclusions and silicate inclusions (such inclusions may be hereinafter referred to as A-type inclusions). As for sulfide inclusions, they are represented by MnS in the usual cases; and as for silicate inclusions, they are represented by MnO.SiO.sub.2. These two types of inclusions are so highly plastic that they stretch slenderly in the rolling direction during hot-rolling operations, thus resulting in poor cold formability and toughness in the transverse direction of the rolling.
Therefore, the recent study and development efforts for improving cold formability and toughness, are directed to (1) sulfide shape control, which means the conversion of stringer MnS into global sulfide by adding Zr, rare earth metals or the like having strong coherence with S, (2) a special steel making practice to reduce the content of S to less than 0.005 %, thereby reducing the content of MnS, and (3) the decrease of silicate inclusions by the addition of a deoxidizer stronger than Si (such as Al). Each of these means are expensive and undesirable for industrial use.